This invention relates to wellheads and to apparatus providing capability for removal and insertion of wellhead stripper rubbers through a blowout preventer, such apparatus being more particularly useful as a workover tool for conversion of existing wellheads to units having enhanced reworking capabilities.
Superatmospheric pressures exist, or may be suddenly encountered, in many wells, such as oil and gas wells, and accordingly drilling and producing operations must be carried out while the pressure in the well bore is confined. Blowout preventers are used on many wells which include devices being capable of sealing the annular space between an inner and an outer pipe or casing. Such blowout preventers are not a permanent portion of a wellhead and if wells are under pressure, then blowout preventers are positioned on a wellhead prior to reworking of the well, which blowout preventers are subsequently removed and used on another well to be reworked.
When reworking is required on a well, for example of the type requiring removal of a tubing string, wells with pressure not having an effectual stripper rubber must either be provided with a stripper rubber or killed. If a well with pressure or having the possibility of being with pressure is to be reworked, a blow out preventer is also utilized during the reworking operation. Installation of a stripper rubber in a wellhead requires killing of the well unless the stripper rubber can be installed through an in place blowout preventer. Killing a well, that is, the process of feeding a fluid such as water down into the well bore to provide a pressure head, is undesirable as a result of the time and expense involved in the operation. Additionally, there is an ever present possibility that a well which has been killed cannot be revived, ending its useful production life.
Even when blowout preventers are used, if a stripper rubber is unavailable the operation can be undesirably time consuming and cause wear on the components involved. Proper pulling of a tubing string through a blowout preventer on a live well requires having a stripper type apparatus. Without a stripper apparatus such pulling involves, for example, opening of a bottom one of a pair of rams, movement of the tubing collar passed the open bottom ram, closing of the open ram and opening of an upper of the pair of rams, and further upward movement of the tubing coupling through the upper ram which is subsequently closed. This operation must be continuously repeated.
Pulling of tubing is preferable with a stripper rubber in place, as the stripper rubber functions to retain pressure by sealing against the tubing, and it also performs a cleaning function, stripping deposits from against the tubing. Only when the last joint of tubing is pulled uwpardly through the blowout preventer must the sequential opening and closing of the pair of rams be utilized. During pulling of the balance of the tubing string, contact between the stripper rubber and tubing, including the coupling, retains the pressure below the stripper rubber. In this operation, however, the stripper rubber is subjected to high wear as each coupling is pulled through the stripper rubber, continually flexing the stripper rubber. Because of this wear, the stripper rubber needs to be removed and replaced as part of the reworking operation.
In most producing wells removal of the stripper rubber is difficult, requiring killing of the well. Although some wellheads, and particularly many so called flange type wellheads, allow removal of a stripper rubber through an in place blowout preventer so that killing of the well is not required, a large number of commercially producing wellheads do not provide such removal and replacement capabilities. In particular, the most common of the so called threaded or screwed type wellheads have included a design whereby a stripper rubber is seated in a casing or tubing body or head and a stripper attachment threadedly attached to the head includes an interior shoulder overlapping the top surface of the stripper rubber. Thus, the stripper rubber can only be removed from the head subsequent to removal of the attachment from the head, which cannot be performed with a blowout preventer in place due to the configuration of the attachment. Accordingly, wells of this type must be killed for proper repair or other reworking activities. Prior to the instant invention, the capability for a relatively simple manner in which to convert the large number of field operating production screwed type wellheads to units allowing stripper rubber insertion or removal though a blowout preventer has not existed.
It is thus desirable to provide wellhead apparatus which allows reworking of the wellhead without requiring killing of the well. It is also desirable to provide such structure which allows removal and insertion of a stripper rubber into the wellhead through an in place blowout preventer. It is further desirable to provide a method whereby existing production wells not having the capability for stripper rubber replacement through an in place blowout preventer can be readily modified to have such capability. As flanged wellheads, compared to screwed wellheads, are particularly expensive, generally heavier and more massive then screwed wellheads, it is particularly desirable that such method and apparatus be available with screwed, as opposed to flanged, wellhead apparatus.